Solvent extraction is an important part of many processes for the recovery of precious metals from their ores (e.g. ore concentrates) or from scrap material. Solvent extraction can be employed to separate precious metals from base metals and other substances, and from each other, in order that relatively pure metal samples may be recovered.
In order to achieve this, typically an aqueous acidified solution comprising species of two or more different precious metals, optionally in combination with base metals, is contacted with an organic phase comprising an extractant. Typically, the extractant is selective for one or more of the precious metals to be separated, thus facilitating their separation by selectively extracting them from the aqueous phase into the organic phase. Further processing steps enable recovery of the separated metal.
For example, GB 1 495 931 describes organic solvent extraction of platinum and iridium species from an aqueous acidic solution also containing rhodium species by using a solvent containing a tertiary amine extractant. However, this separation does not achieve separation of the metals in the presence of palladium species, and so has the disadvantage of requiring palladium species to be removed before platinum may be liberated.
EP 0 210 004 describes an extractant which is suitable for extracting platinum from an acidified aqueous solution which also includes palladium. The extractant is a mono-N-substituted amide. This extractant also permits separation of platinum species from other precious metals which may be present in the solution, particularly where ruthenium, iridium and osmium species are present in oxidation state III, while the platinum species is in oxidation state IV. EP 0 210 004 explains that this may be achieved by treating the aqueous phase with a mild reducing agent. Following treatment with the mono-N-substituted amide, further treatment of the aqueous phase is required if the palladium is to be recovered.
Palladium may be extracted into an organic phase using thioether extractants. For example, as explained in US2009/0178513, DHS (di-n-hexylsulfide) is one of the most commonly used industrial extractants for palladium, which is capable of selectively extracting palladium from an acidic aqueous solution containing palladium, platinum and rhodium. US2009/0178513 proposes a different thioether-containing extractant having the following formula:
where R1, R2 and R3 each represents a group selected from a chain hydrocarbon group having 1 to 18 carbon atoms. US2009/0178513 states that the extractant described therein enables the extraction of palladium to be performed more rapidly than is possible using DHS, but that the other platinum group metals (including platinum) are hardly extracted at all. The palladium in the organic solution is recovered using ammonia.
As an alternative to selective extraction, some documents propose simultaneously extracting more than one metal into the organic phase, followed by selectively removing each metal from the organic phase. For example, U.S. Pat. No. 4,654,145 describes co-extraction of precious metals including gold, platinum and palladium into an organic phase using Kelex® 100:

The gold is then precipitated out of the solution, followed by precipitation of the palladium. Platinum is removed from the organic phase by washing with an aqueous phase. However, the processes proposed in this document suffer the disadvantage of including precipitation to separate the metals extracted into the organic phase.
U.S. Pat. No. 5,045,290 describes a process for the recovery of Pt and Pd from an impure substantially gold-free precious and base metal-bearing acidic chloride or mixed chloride/sulphate solution, comprising the steps of contacting the acidic solution having a pH of less than about 1.5 with an organic solution comprising an 8-hydroxyquinoline solvent extraction reagent, a phase modifier and an aromatic diluent to extract simultaneously platinum and palladium into the organic solution, scrubbing the co-extracted solution to remove co-extracted impurities and acid, stripping the loaded organic with a buffer solution operating in the pH range 2-5 at 20-50° C. to selectively recover the platinum, stripping the platinum-free loaded organic with 3-8 M hydrochloric acid to recover the palladium, and regenerating the organic solution by washing with water.
Guobang et al. (Reference 1) describes co-extraction of Pt and Pd using petroleum sulfoxides. After washing, Pt is removed from the organic phase using dilute HCl and Pd is removed using aqueous NH3.
US2010/0095807 describes a separation reagent for separating platinum group metals from an acidic solution containing rhodium, platinum and palladium. The reagent has the general formula:
wherein at least one of R1, R2 and R3 represent an amide group represented by:
wherein each of R1 to R3 other than the amide group, and R4 to R6 are hydrocarbon groups. In the separation methods described in this document, rhodium, platinum, and palladium are co-extracted using the extractant reagent. Highly concentrated hydrochloric acid solution is then used to recover rhodium from the organic phase. The platinum and palladium are then back-extracted from the organic phase using highly concentrated nitric acid solution, to produce an aqueous solution including both platinum and palladium.
U.S. Pat. No. 4,041,126 describes co-extraction of platinum and palladium from acidic aqueous medium using an organically substituted secondary amine capable of forming complexes of platinum and palladium. Palladium is selectively recovered from the organic phase with an aqueous solution of an acidified reducing agent. Platinum is separately recovered using an alkaline stripping reagent selected from alkali metal and alkaline earth metal carbonates, bicarbonates and hydroxides.